Leakage inspectors that use air pressure to check whether containers or mechanical parts have a leak, such as gas meters, fuel tanks of cars, and engine housings, have been practically used (see Patent Literature 1, for example). The leakage inspectors that have been used conventionally can be divided into two types. In one type (hereinafter called pressure-difference leakage inspectors), air pressure is applied at the same time to the inside of a device under inspection, such as the container or mechanical part described above, and to the inside of a reference tank which has no leak, and it is determined whether or not the device has a leak according to whether a pressure difference occurs between the insides of the two units described above. In the other type (hereinafter called gauge-pressure leakage inspectors), air pressure is applied only to the inside of a device under inspection, and it is determined whether or not the device has a leak according to whether the applied air pressure changes within a predetermined period of time.
[Conventional Pressure-difference Leakage Inspectors]
FIG. 1 is a diagram showing the structure of a leakage inspector 100 that is a conventional pressure-difference leakage inspector.
The pressure-difference leakage inspector 100 comprises a pneumatic apparatus 200 and a decision apparatus 300.
The pneumatic apparatus 200, shown in FIG. 1, includes a pneumatic source 201 for applying pressure to the inside of a device under inspection, such as a compressor; a pressure control valve 202 for controlling the amount of air output from the pneumatic source 201 to control the air pressure which the pneumatic source 201 externally applies to a predetermined air pressure; a three-way solenoid valve 203 capable of switching between a state (XY-port connection state) in which the air pressure controlled by the pressure control valve 202 is applied to devices A and B and to a reference tank 207 and a state (YZ-port connection state) in which air in the devices A and B and the reference tank 207 is discharged to the atmosphere; sealing valves 204A and 204B for sealing the inside air in a state in which the air pressure is applied to the inside of the device A or B and to the inside of the reference tank 207; a differential pressure gauge 205 for measuring a pressure difference between the inside of the device A or B and the inside of the reference tank 207; switching valves 206A and 206B for switching between the devices A and B to which the air pressure is applied to allow one of the devices to be inspected while the other is replaced; the reference tank 207; and connection jigs 208A and 208B for connecting air supply lines to the devices A and B. One end of the air supply lines is connected to the outlet of the pneumatic source 201, the air supply lines supply air to the devices A and B and to the reference tank 207. As shown in FIG. 1, the pressure control valve 202, the three-way solenoid valve 203, the sealing valves 204A and 204B, and the switching valves 206A and 206B are disposed in the air supply lines.
The decision apparatus 300 includes a variable-gain amplifier 301 (with the gain being switched between a low gain and a high gain) for amplifying the output signal of the differential pressure gauge; an A/D converter 302; a microcomputer including an input port 303, a CPU (central processing unit) 304, a ROM (read-only memory) 305, a RAM (random access memory) 306, which is a memory to and from which data can be written and read, and an output port 307; and a leak decision display unit 308 for showing a leak decision result, such as a display unit.
In the current case, the ROM 305 stores an operation-timing generation program, a control-information generation program, a measured-value storage program, and a leak decision program that make the microcomputer operate as operation-timing generation means, control-information generation means, measured-value storage means, and leak decision means, respectively. These programs are read from the ROM 305 and stored in the RAM 306 as an operation-timing generation program 306A, a control-information generation program 306AB, a measured-value storage program 306B, and a leak decision program 306C when the microcomputer is started. In the drawings, programs are abbreviated as “PGs”. The programs stored in the RAM 306 are read by the CPU 304, and the CPU 304 decodes and executes them to function as the above-described means.
FIG. 2A to FIG. 2E are graphs for describing a general operation of the leakage inspector 100. FIG. 2A is a graph showing temporal changes of the output of the variable-gain amplifier 301. In FIG. 2A, the vertical axis indicates the output of the variable-gain amplifier 301 and the horizontal axis indicates time. FIG. 2B is a graph showing a timing signal C1 that is H (high) during a pressure applying period T1 and is L (low) in the other periods. FIG. 2C is a graph showing a timing signal C2 that is H during a stable period T2 and is L in the other periods. FIG. 2D is a graph showing a timing signal C3 that is H during an inspection period T3 and is L in the other periods. FIG. 2E is a graph showing a timing signal C4 that is H during an air discharging period T4 and is L in the other periods. In each of FIG. 2B to FIG. 2E, the vertical axis indicates the voltage of a corresponding control signal and the horizontal axis indicates time.
The pneumatic apparatus 200 operates differently in four transition periods of the pressure applying period T1, the stable period T2, the inspection period T3, and the air discharging period T4.
In the pressure applying period T1, the port X and the port Y are connected in the three-way solenoid valve 203, and the sealing valves 204A and 204B are made to open. With these actions, air pressure caused by the operation of the pneumatic source 201 is applied to the inside of either the device A or B and to the inside of the reference tank 207.
In the stable period T2, the sealing valves 204A and 204B are closed. With these actions, the inside of either the device A or B and the inside of the reference tank 207 are sealed with the air pressure being applied. This state is held for a predetermined period to make the inside air pressure stable (to remove the influence of adiabatic changes in the air pressure). In the stable period T2, the gain of the variable-gain amplifier 301 is switched to the low gain. The decision apparatus 300 determines that the device A or B has “no large leak” if the output VM (in FIG. 2A) of the variable-gain amplifier 301 does not reach a setting (NG) in this state. The decision result is shown in the leak decision display unit 308. When the stable period T2 is finished, the output of the variable-gain amplifier 301 is reset to zero, and the gain of the variable-gain amplifier 301 is switched to the high gain. Then, the period proceeds to the inspection period T3.
In the inspection period T3, the pressure difference output from the differential pressure gauge 205 is amplified by the variable-gain amplifier 301, which is set to have the high gain. Whether a leak exists is determined by whether the amplified value output from the variable-gain amplifier 301 exceeds the setting (NG). The decision result is shown on the leak decision display unit 308. In the stable period T2, the air pressure inside either the device A or B and the air pressure inside the reference tank 207 are stable. In the inspection period T3, the pressure difference amplified by the variable-gain amplifier 301, set to have the high gain, is checked to detect even a small change in the pressure difference.
In the air discharging period T4, the sealing valves 204A and 204B are made to open, and the port Y and the port Z of the three-way solenoid valve 203 are connected. With these actions, the air sealed inside either the device A or B and the air sealed inside the reference tank 207 are discharged to the atmosphere through the port Z, and the inside air pressure becomes equal to the atmospheric pressure, thus completing the inspection.
Switching to each of these periods is conducted, for example, as described below. First, the operation-timing generation means generates one of the timing signals C1, C2, C3, and C4 (FIG. 2B to FIG. 2E) corresponding to the current period. The control-information generation means generates control signals that make the three-way solenoid valve 203, the sealing valves 204A and 204B, and the variable-gain amplifier 301 execute the actions corresponding to the period indicated by the signal, that is, one of the timing signals C1, C2, C3, and C4, generated by the operation-timing generation means. The generated control signals are output from the output port 307 to the three-way solenoid valve 203, the sealing valves 204A and 204B, and the variable-gain amplifier 301. The three-way solenoid valve 203, the sealing valves 204A and 204B, and the variable-gain amplifier 301 perform the actions according to the control signals in each period.
The reference tank 207 of the pressure-difference leakage inspector 100 should have better air temperature stability than the device A or B. When a test pressure TP is applied to the inside of a device under inspection and to the inside of the reference tank 207, even if the temperature of the supplied air is room temperature, the temperature inside the device and the temperature inside the reference tank 207 increase (adiabatic characteristics). These temperature increases depend on the test pressure TP and the supplied-air temperature.
Since the internal pressure of the device under inspection equals the internal pressure of the reference tank 207 at the end of the pressure applying period T1, the pressure difference is almost zero. Because the reference tank 207 has better air temperature stability than the device under inspection, however, the air temperature becomes stable in the reference tank 207 more quickly than in the device under inspection after the sealing valves 204A and 204B are closed. As a result, a change in air temperature in the device under inspection appears as a change in pressure difference. When the device under inspection and the reference tank 207 have no leak, the pressure difference attenuates as time passes and reaches a certain pressure difference after a while. This is the reason why, when the sealing valves 204A and 204B are closed, the device under inspection and the reference tank 207 have a pressure difference even though they have no leak.
[Conventional Gauge-pressure Leakage Inspectors]
FIG. 3 is a diagram showing the structure of a leakage inspector 110 that is a conventional gauge-pressure leakage inspector. In FIG. 3, the same symbols as those used in FIG. 1 are assigned to the same portions as those shown in FIG. 1.
The gauge-pressure leakage inspector 110 comprises a pneumatic apparatus 400 and a decision apparatus 300. Since the decision apparatus 300 is the same as that in the pressure-difference leakage inspector 100, only the structure of the pneumatic apparatus 400 will be described here.
The pneumatic apparatus 400 includes a pneumatic source 201, a pressure control valve 202, a three-way solenoid valve 203, a sealing valve 204, switching valves 206A and 206B, connection jigs 208A and 208B, and a pressure gauge 209 for measuring the pressure inside device A or B.
FIG. 4A is a graph showing changes in the pressure measurement value output from the pressure gauge 209. In FIG. 4A, the vertical axis indicates the output of the pressure gauge 209 and the horizontal axis indicates time.
In the leakage inspector 110, a test pressure TP is applied to the inside of the device A or B in a pressure applying period T1. The sealing valve 204 is closed at the end of the pressure applying period T1. After the sealing valve 204 is closed, the air pressure inside the device A or B is gradually reduced due to an adiabatic change (the heat of the air inside the device, where the temperature was increased by applying the pressure, is gradually discharged to the device and the air temperature decreases to change the air pressure).
FIG. 4B is a graph showing the output waveform of a variable-gain amplifier in the decision apparatus 300.
The output of the variable-gain amplifier is obtained by amplifying the difference between the test pressure TP, which is a bias value, and the pressure shown in FIG. 4A. In a stable period T2, the variable-gain amplifier operates with its gain set to a low gain, as in the pressure-difference leakage inspector 100. When the output of the variable-gain amplifier reaches a setting (NG), the decision apparatus 300 determines that the device A or B has “a large leak”. If the output of the variable-gain amplifier does not reach the setting NG during the stable period T2, the output of the variable-gain amplifier is reset and the gain of the variable-gain amplifier is switched to a high gain. Then, the period proceeds to an inspection period T3.
In the inspection period T3, the variable-gain amplifier of the decision apparatus 300 operates with the high gain. When the output M of the variable-gain amplifier, corresponding to a reduction in pressure, does not exceed the setting (NG) during the inspection period T3, the decision apparatus 300 determines that the device under inspection has “no leak”, thus completing the inspection. The same method as that in the pressure-difference leakage inspector is used to control each period (as shown in FIG. 4C to FIG. 4F).
[Malfunction of Leakage Inspector]
The operations of the leakage inspectors described above apply when each part of the leakage inspectors operates normally. A malfunction may occur in some cases. In those cases, while the malfunction is not detected, the inspection may be continued to determine that all devices under inspection have “no leak” or that all devices under inspection have “a leak”, irrespective of whether the devices under inspection actually have a leak or not.
Example malfunctions in the parts will be described below.
(1) It is assumed that the pressure-difference leakage inspector 100 (FIG. 1) performs leak inspection while the port X and the port Y are not connected in the three-way solenoid valve 203 or while both the sealing valves 204A and 204B are closed. In that case, air pressure is not applied to the inside of the device A or B or the inside of the reference tank 207. Therefore, the pressure difference between both the insides is zero, and the value measured by the differential pressure gauge 205 is also zero. As a result, the leak decision means may incorrectly determine that the device under inspection has “no leak”.
(2) It is assumed that the pressure-difference leakage inspector 100 (FIG. 1) performs leak inspection while both the switching valves 206A and 206B are closed, whereas the three-way solenoid valve 203 and the sealing valves 204A and 204B operate normally. In that case, since there is usually no leak in the air supply lines, the value measured by the differential pressure gauge 205 is sufficiently small. As a result, the leak decision means may incorrectly determine that the device under inspection has “no leak”.
The same malfunction also occurs in the gauge-pressure leakage inspector 110 (FIG. 3).
(3) It is assumed that the pressure-difference leakage inspector 100 performs leak inspection while the differential pressure gauge 205 is inoperable. In that case, since the differential pressure gauge 205 outputs a value of zero, the leak decision means may incorrectly determine that the device under inspection has “no leak”, irrespective of the conditions of the devices A and B.
If one of the above types of malfunctions occurs in the gauge-pressure leakage inspector 110, the value measured by the pressure gauge 209 is zero. In that case, the leak decision means may incorrectly determine that the device under inspection has “no leak”, irrespective of the conditions of the devices A and B.
To solve the drawbacks of the leakage inspectors described above, the applicant proposed a pressure-difference leak tester having a self-diagnosis function (see Patent Literature 2).
The pressure-difference leak tester having a self-diagnosis function, proposed before, determines before the start of an inspection that the leak tester operates normally when control is performed such that the port X and the port Y of the three-way solenoid valve 203 are connected while one of the sealing valves 204A and 204B is closed, and the value obtained by amplifying the detected pressure difference in the variable-gain amplifier with the low gain exceeds the NG level. In other words, when control is performed such that the port X and the port Y of the three-way solenoid valve 203 are connected while one of the sealing valves 204A and 204B is closed, if the three-way solenoid valve 203 operates normally, pressure is applied only to the device under inspection or to the reference tank 207. When the differential pressure gauge 205 operates normally and the decision apparatus 300 also operates normally in that state, a decision result should be output showing that the leak tester operates normally because the absolute value of the pressure difference becomes equal to or larger than the NG level even with the low-gain amplification sensitivity. Therefore, if a decision result indicating that the device under inspection has a leak is not output in that condition, it can be determined from the decision result that something malfunctions.
[Patent Literature 1] Japanese Registered Patent No. 1775588
[Patent Literature 2] Japanese Patent Publication No. H7-101193